Sometimes you might find your main board full and without space to produce items and keep making game progress. If that happens you can do one of two things:
Save any valuable items by dragging them into your inventory on the bottom left of the screen - make sure to keep track of the amount of storage space you have as it is limited. More storage slots can be purchased with Diamonds.
Sell an item by tapping on it and then on the sell 💲 button on the bottom right of the screen. Each item sold will grant Coins according to its level. First-stage items cannot be sold, however, and those can only be discarded by tapping on them and then on the trash button which appears.
After selling an item, the sell 💲 button will turn into an “Undo” button which can be tapped to return the item to your main board at the cost of the Coins received for selling it. Once you continue playing, the undo option will not be available anymore so make sure to keep a close eye on the items you sell or discard!